Superheated water might remain still, and when you remove the container from the microwave or add an item like a spoon or a tea bag, it can cause the water to boil explosively. This sudden eruption of steam can lead to burns and scalding injuries. https://sciencenotes.org/superheating-water-in-a-microwave-oven/
also:
https://www.snopes.com/fact-check/boil-on-troubled-waters/
example: https://www.youtube.com/watch?v=LpDs7Xm1uLo&t=1